This invention relates generally to fluid separation and, more particularly, to fluid separation via one or more membrane separation element assemblies. A variety of commercial processes rely on fluid separation techniques in order to separate one or more fluid components from a mixture. In particular, various such processes may involve the separation of liquid mixtures, the separation of vapors or gases from liquids, or the separation of intermingled gases.
For example, carbon dioxide, commonly categorized as an acid gas, is frequently found in natural gas streams, such as in levels as high as 80%. In combination with water, carbon dioxide can be highly corrosive and rapidly damage or destroy pipelines and associated processing equipment. Consequently, such processing typically has required either the use of exotic and more costly materials of construction or the effective isolation or removal of carbon dioxide from the process stream.
While a variety of acid gas removal technologies, including absorption processes, cryogenic processes, adsorption processes are available, fluid separation via the use of membranes, e.g., thin semipermeable membranes that selectively separate some fluid compounds or constituents from others, has experienced increased popularity for such and various other fluid separation applications. Such membrane separations are generally based on relative permeabilities of various components of the fluid mixture, resulting from a gradient of driving forces, such as pressure, partial pressure, concentration and temperature. Such selective permeation results in the separation of the fluid mixture into portions commonly referred to as “retentate”, e.g., generally composed of slowly permeable components, and “permeate”, e.g., generally composed of faster migrating components.
Gas separation membranes are commonly manufactured in one of two forms: flat sheet or hollow fiber. The flat sheets are typically combined into a spiral wound element, whereas the hollow fibers are commonly bundled together in a manner similar to a shell and tube heat exchanger.
In typical spiral wound arrangements, two flat sheets of membrane with a permeate spacer in between are joined, e.g., glued, along three of their sides to form an envelope, i.e., a “leaf”, that is open at one end. Many of these envelopes are separated by feed spacers and wrapped around or otherwise to form a permeate tube with the open ends of the envelopes facing the permeate tube. Feed gas enters along the side of the membrane and passes through the feed spacers separating the envelopes. As the gas travels between the envelopes, carbon dioxide and other highly permeable compounds permeate into the envelope. These permeated components have only one available outlet: they must travel within the envelope to the permeate tube. The driving forces for such transport is the low permeate and the high feed pressures. The permeate gas enters the permeate tube, such as through holes passing though the tube. The permeate gas then travels through the tube to join the permeate from other of the tubes. Gas on the feed side that does not permeate, leaves through the side of the element opposite the feed position.
In hollow fiber elements, very fine hollow fibers are wrapped around a central tube in a highly dense pattern. In such a wrapping pattern, both ends of the fiber end up at a permeate pot on one side of the element. Feed gas flows over and between the fibers, and some components permeate into them. The permeated gas then travels within the fiber until it reaches the permeate pot, where it mixes with permeates from other of the fibers. The total permeate exits the element through a permeate conduit or pipe. The gas that does not permeate eventually reaches the central tube of the element, which is typically perforated similar to the permeate tube in the spiral wound element. In this case, however, the central tube is for residual collection, not permeate collection.
As will be appreciated, each element type has certain advantages. For example, spiral wound elements typically can handle higher pressures, are more resistant to fouling and have a long history in service in natural gas sweetening. In contrast, hollow fiber elements typically have a higher packing density. Consequently, hollow fiber-based installations are typically smaller than a corresponding spiral wound installation.
In either case, the membranes, once produced into elements, are typically formed into modules, e.g., a tube containing a plurality of membrane separation elements. Modules can be used singly or, more commonly, interconnected in series or parallel arrangements or arrays. Typically, an installation may have from at least two up to several hundred modules in an array. Each module will have an input (e.g., feed) stream, an output or residual stream that contains the substances which did not pass through the membrane separation element and a permeate stream contains the substances which pass through, e.g., permeate through, the membrane separation element.
Many such separation applications require rather high pressures. In many instances the pressures in such processes are in the range of about 3447 to about 20684 kPa (500 to 3000 psi). In dealing with such pressures, besides having sufficient wall thickness, it is necessary to have good pressure seals. The various process flow streams (e.g., feed, residual and permeate streams) must remain properly separated. Any intermingling of these streams decreases the efficiency of the process.
Current spiral wound membrane gas separation pressure vessel configurations are a product of the reverse osmosis industry, where such configurations have been their standard from many years. This vessel concept was designed for applications where filament wound plastic tubes could be easily incorporated, e.g., the internal diameter of the plastic tube could be easily manufactured to exact dimensions as the membrane web was wound over a mandrel.
The gas membrane industry has needed to modify the standard reverse osmosis vessel configuration to meet or satisfy their unique process requirements. For example, they could incorporate larger permeate pipes or conduits in order to better handle larger possible permeate flows. Further, in order to meet the typical high pressure operation requirements associated with the gas processing industry and the tight roundness and diameter specifications required for such processing, steel tubing has been employed and with the internal diameter of the steel tubing honed to an appropriately high surface finish, e.g., 16 RMS or less.
In addition, an end draw configuration, such as used in reverse osmosis processing, has also typically been utilized in gas membrane separation applications. Such end draw configurations typically result in the permeate header extending out from the ends of the pressure tubes. Thus, such configurations generally result in a membrane skid being made longer than otherwise desired or in a reduction in length of the membrane tubes used therein, with a corresponding reduction in the capacity of the unit.
One of the difficulties in building such membrane skids is the need to ensure that the permeate header lines up perfectly with the flange connections at the end of the membrane pressure tube. Moreover, the permeate header must typically be removed every time membrane elements are loaded and unloaded in the tube. In use, such membrane module assemblies may be subjected to repeated pressurization and depressurization cycles due to periodic equipment maintenance and day-to-day processing needs requiring equipment shut-down. However, the seals of a module may not be capable of elastically responding to a rapidly decreasing pressure when the module is depressurized sufficient to maintain a reliable seal with or in conjunction with associated membranes. As a result, it is likely that the seals may become displaced and/or lose sealing contact during depressurization which, in turn, causes fluid leakage to occur if the module is subsequently attempted to be repressurized. Such fluid leakage will thus cause the module to fail to perform its intended fluid-separation functions. Once fluid leakage has occurred, therefore, the only alternative is to remove the module from service and rebuild the membrane seal components.
In addition, a common problem associated with the use of spiral wound membranes is that each module containing the membranes is typically required to be machined to close tolerances in order to assure good pressure seals. As a result, cost for each module can be significantly increased.
Thus, there is a need and a demand for improved assemblies for use in separation of a fluid feed via a plurality of membrane separation elements.
There is also a need and a demand for improved membrane separation assemblies that permit permeate fluid removal from a location other than the end center of the tube. In particular, there is a need and a demand for improved membrane separation assemblies that permit permeate removal from both ends of a membrane string. Further, for example, there is a need and a demand for improved membrane separation assemblies that permit permeate removal from one or more desired locations intermediate to the ends of the tube.